WASHINGTON (AP) - Boeing mechanics are to blame for accidental contamination of oxygen systems aboard one of the modified Boeing 747 planes known as Air Force One used for presidential travel, the Air Force said in an investigation report.

The plane was undergoing extensive maintenance at a Boeing facility in San Antonio, Texas, when the mishap occurred in April 2016. No one was injured.

An Air Force accident investigation report released this month faulted three Boeing mechanics for supplying and using tools and parts that did not meet cleanliness standards while checking oxygen lines for leaks. The standards are meant to prevent a fire hazard. The mechanics also used an unauthorized cleaning procedure.

The report said Boeing has paid more than $4 million to clean or replace contaminated portions of the aircraft’s oxygen system. The plane has been returned to service with the 89th Airlift Wing at Andrews Air Force Base in Maryland.

The investigators also concluded that Boeing failed to adequately oversee the quality of maintenance being performed on the aircraft, which is a Boeing 747-200B specially configured for aerial refueling and equipped with special communications gear.